1. Technical Field
The present invention relates to an image processing device having an inkjet head, and to a method of detecting missing dots in the image processing device.
2. Related Art
An inkjet printer that prints by ejecting ink onto the recording medium is one type of image processing device known from the literature. The ink nozzles of an inkjet printer can become clogged when ink dries in a nozzle or the printer is subject to vibration, resulting in the inkjet printer not ejecting ink normally. When this happens, ink does not land on the recording medium and white bands, for example, may be formed in the printed image (characters or graphics) due to the missing dots, resulting in a drop in image quality.
JP-A-2006-35727 teaches a method of checking whether or not ink is discharged normally from the inkjet head set (each nozzle of the inkjet head) when printing images with an inkjet printer (referred to herein as “missing dot detection”). The inkjet printer taught in JP-A-2006-35727 inspects each nozzle of each inkjet head nozzle row to determine if the nozzle is discharging normally by printing a test pattern on the recording medium with the inkjet head set, reading the test pattern with a line CCD, and comparing the read test pattern with inspection data previously stored in memory.
Checking for missing dots nozzle by nozzle using a test pattern as described above can prevent missing dots resulting from ink ejection problems and improve print quality. However, there are situations in which a printing problem (misprint) is detected only if so many dots are missing that characters cannot be read (if there are many consecutive missing dots), and a certain number of missing dots in the printout is acceptable (do not need to result in a misprint determination) if the characters can be read. In this situation, inspecting for missing dots nozzle by nozzle in each inkjet head nozzle row as described in JP-A-2006-35727 is inefficient.
Furthermore, because inspection using a test pattern is conducted at scheduled times, such as before the printing process starts, noticing that dots are missing (that there is an ink ejection problem) may not be possible until the user sees the actual printout when dots start missing in the middle of a long continuous printing process, for example. If a large volume of printing has already been completed when missing dots are noticed after the printing process starts, the printing process must be repeated, thus wasting time and money.
JP-A-2006-35727 teaches inspecting for ink ejection problems by printing a test pattern on a recording medium with the inkjet head set, reading the test pattern with a line CCD, and comparing the read test pattern with inspection data previously stored in memory. However, JP-A-2006-35727 is silent about technology related to the printing position shifting when printing the test pattern with the inkjet head set. Conveyance (paper feed) of the recording medium may not be consistent, and the printing position of the test pattern may be shifted from the expected printing position (the position specified by the printer driver), when printing with the inkjet head.
As a result, the inspection method taught in JP-A-2006-35727 may analyze an area offset from the expected printing position due to deviation in the position of the printed area, resulting in false detection of missing dots (a misprint) even though ink is discharged (ejected) normally.